The present invention relates to a communication line driver and, more particularly, to a technique which is useful when it is applied to, for example, an LSI for interface and a communication terminal apparatus in an integrated service digital network (ISDN).
The ISDN makes it possible to perform a high-speed digital transmission in which a traffic rate is equal to 144 kbps comprising two channels of a B (data) channel of 64 kbps and a D (control) channel of 16 kbps by using a subscriber's line which has conventionally been used for an analog audio signal transmission and is also called a telephone line. In such an ISDN, the subscriber's line is terminated in a digital service unit and is connected to an S/T point as a user network interface.
FIG. 4 shows a conventional S/T point driver. In such a driver, a transformer in which a turns ratio is equal to 2.2:1 is applied. One end of the transformer is connected to a center voltage point as a voltage division using resistors R1 and R2 and the other point is connected to a switch of a bipolar transistor. By an input of a digital signal, a pnp bipolar transistor Q2 or an npn bipolar transistor Q4 is turned on. When the pnp bipolar transistor Q2 is turned on, a high voltage side power source of 5 V is applied to the transformer. When the npn bipolar transistor Q4 is turned on, one end of the transformer is connected to a grand GND. By such a method, pulses of both positive and negative polarities are output by changing the direction of a current which flows in the transformer. Resistors R3 and R4 are connected to emitter electrodes of the transistors Q2 and Q4, respectively, and the connecting point is connected to base electrodes of bipolar transistors Q1 and Q3 of the same polarity, respectively. Collector electrodes of the transistors Q1 and Q3 are connected to base electrodes of the bipolar transistors Q2 and Q4, respectively. When an output current becomes too large, a voltage drop of the resistor R1 or R2 increases. Due to this, a base voltage of the bipolar transistor Q1 or Q3 increases, so that an on resistance of the bipolar transistor Q1 or Q3 decreases and the voltage applied to the base of the bipolar transistor decreases. Consequently, the circuit is controlled so that the output current decreases and functions as a current limiting circuit.
FIG. 5 shows another conventional example of an S/T point driver.
A pulse (`0` signal) of both polarities is transmitted by changing the direction of the current which flows in a transformer connected to terminals 01 and 02 in dependence on whether MOS transistors T1 and T4 are turned on or MOS transistors T2 and T3 are turned on by a bridge circuit of the MOS transistors T1 to T4. The above operation is equivalent to the operation of an equivalent circuit shown in FIG. 6A. The `1` signal is transmitted in a state in which all of the MOS transistors T1 to T4 are turned off. Source electrodes of the MOS transistors T2 and T4 are commonly connected at a terminal 04 and are connected to the ground GND by an externally attached resistor Rm. Gate electrodes of the MOS transistors T2 and T4 are connected to an output terminal of a differential amplifier OA. The source electrodes of the MOS transistors T2 and T4 are connected to an inverting input terminal (-) of the differential amplifier OA. A reference voltage Vref is applied to a non-inverting input terminal (+). When the MOS transistor T2 or T4 is turned on, a voltage/current converting circuit of (OA, T2, and Rm) or (OA, T4, and Rm) is formed. The potential of the terminal 04 is equal to the reference voltage Vref by the voltage follower operation of the differential amplifier OA. The current flowing in the resistor Rm is a constant current of IR=Vref/Rm. There is a specification of (CCITT recommendation I.430 corresponding to three kinds of load conditions (50 .OMEGA., 400 .OMEGA., 5.6 .OMEGA.) as for a pulse amplitude at the S/T point. In this method, as shown in FIG. 6B, the characteristics of the output to the load resistor in case of using the transformer of the turn ratio of 2:1 are shown in FIG. 7. A potential difference Vd1 between the terminals 03 and 04 is equal to 2.4 V and is constant for the load resistance. A potential difference Vd2 on the secondary side of the transformer is equal to 1.2 V and is constant to the load resistance. In order to satisfy the specification of the pulse mask, it is necessary to set a resistor RI which is externally attached to 30 .OMEGA. and a resistor RI' in FIG. 6B to 60 .OMEGA.. An output impedance of the driver is equal to 30 .OMEGA. and satisfies the specification of 20 .OMEGA. or more. When a load resistor RL is set to 5.6 .OMEGA. and 50 .OMEGA., it is controlled by a constant current (IR=16.5 mA) and an output voltage is equal to Vout=IR.times.RL. When the load resistance is equal to 5.6 .OMEGA., the output voltage is equal to 84 mV, which is lower than the specification value of 150 mV or less. When the load resistance is equal to 50 .OMEGA., the output voltage is equal to 0.75 V, which lies within a specification range of 0.65 to 0.85 V. Both of those voltage values satisfy the characteristics of the specification. When the load resistance is equal to 400 .OMEGA., the voltage limit is performed and Vout=Vd2.times.RL/(RI+RL). The output voltage is equal to 1.135 V, which lies within a specification range of 0.65 to 1.20 V, so that the output voltage satisfies the characteristics of the specification.
As an example of literatures disclosed with respect to the S/T point driver of the ISDN, there are papers of The Institute of Electronics and Communication Engineers of Japan of NTT (No. 10, Vol. J72-B-I, October, 1989) and JP-A-62-287793 shown in FIG. 5 of the prior art.
In the conventional circuit shown in FIG. 4, since the center point potential is supplied by voltage division using the resistors R1 and R2, the current stationarily flows into the resistors R1 and R2, so that the power consumption increases.
An output impedance of the driver is determined by resistors R3 and R4 connected to the emitters of the bipolar transistors Q2 and Q4. A variation in those resistors results in a variation of the output impedance when it is seen from the line side. In order to satisfy the specification of the output impedance in the ISDN (20 .OMEGA. or more when the `0` signal is transmitted), since it is necessary to adjust the value, it is difficult to provide such resistors R3 and R4 in the LSI. Therefore, it is difficult to make an LSI of the driver.
According to the conventional technique shown in FIG. 5, the resistor Rm having a small temperature dependency is needed to make the current constant. For this purpose, it is necessary to externally attach the resistor Rm. When the transformer of the turns ratio of 2:1 is used, resistors R10 and R20 are set to 60 .OMEGA. (total 30 .OMEGA. as a value in secondary side calculation), and as shown in FIG. 7, an electric power loss by the resistors R10 and R20 at the time of the constant current control when the load resistor is set to 5.6 .OMEGA. and 50 .OMEGA. is relatively large. It causes an increase in electric power consumption of the interface LSI having such a driver.